1. Field of the Invention
The present invention relates to a thermal head for use in thermal transfer type printers or heat-sensitive type printers and more particularly, to a thermal head of the type in which a heat-generating portion is provided on a projecting portion of a substrate.
2. Description of the Related Art
FIG. 18 shows a known thermal printer head. This thermal printer head has an aluminum substrate 1 which is locally covered by a heat-accumulation layer 2 of glass glaze. A heat-generating resistor layer 3 is formed on the heat-accumulation layer 2 and the substrate 1. A conductor layer 4 for supplying electrical current to the heat-generating resistor layer 3 is formed on the layer 3. A heat-generating portion 6, composed of the resistor layer 3 exposed through the conductor layer 4, is formed in a dot-like form. A protective layer 5 is formed on the dot-like heat generating portion 6 to protect the latter from oxidation and wear.
This thermal head is used in contact with a recording medium (not shown) such as an ink ribbon or a heat-sensitive paper. When an electrical current is supplied to the heat-generating portion 6 of the thermal head, heat is generated to transfer an ink in an ink ribbon to the recording medium or enables coloring component in the recording medium to develop a color, whereby information is recorded on the recording medium.
The thermal response characteristic is improved to enable high-speed printing when the glass glaze heat-accumulation layer is thinned as shown in FIG. 19. In such a case, however, the pressure at which the dot-like heat-generating portion 6 is pressed against the ink ribbon is reduced because the amount of projection or height of the heat-generating portion 6 is reduced correspondingly. This causes various inconveniences such as impairment of printing quality and efficiency, as well as increase in the electrical power consumption. Printing quality and efficiency would be improved when the projection height of the heat-generating portion 6 is increased by an increase in the glass heat-accumulation layer 2 as shown in FIG. 20, because in such a case the heat-generating portion 6 can be pressed onto the ink ribbon at higher pressure. In such a case, however, the thermal capacity of the thermal head is increased due to increase in the thickness of the glass glaze heat-accumulation layer 2, with the result that the thermal response characteristic of the thermal head is impaired to, and thus inhibits high-speed printing.